Today, all civilian aircraft are equipped with flight management systems, better known by the acronym FMS. An FMS consists of various functional components which allow the crew of an aircraft to programme a flight on the basis of a navigation database. The FMS calculates lateral and vertical trajectories allowing the aircraft to attain its destination. These calculations are based on the characteristics of the aircraft, on the data provided by the crew and on the environment of the system. The aircraft positioning and guidance functions thereafter collaborate with a view to allowing the aircraft to remain on the trajectories defined by the FMS.
By making it possible to optimize the formulation of the speed strategy of the aircraft, the invention is aimed at improving the possibilities of the said aircraft with a view to allowing the latter to reach particular points at a required time, with maximum precision. This need stems from the exponential increase in air traffic and the corresponding workload of air traffic controllers. Thus, for reasons of safety, but also of economic viability, it is becoming indispensable to impose increasingly strict time constraints on aircraft, notably in the approach phase, at the level of particular points such as a landing runway threshold, a point of convergence of aircraft streams, a heavily frequented crossover point, etc. This may make it possible, for example, to smooth the stream of aircraft before the approach phase.
Today, the FMS of an aircraft calculates optimized flight parameters, with a view to reaching particular points of the flight plan at precise times, in the most effective possible manner and, for example, in an economic manner. With the aim of adhering to these time constraints, the FMS defines a speed strategy.
In the subsequent description and in the claims, the expression “speed strategy” is understood to mean a speed profile assumed to have to be followed by the aircraft, the a priori mission of the guidance module being to determine at any instant of the flight a setpoint speed, that the aircraft seeks to reach, aimed at complying with said speed strategy.
Currently, the FMS of an aircraft consequently carries out calculations of predictions with the aim of complying with a required time of arrival at a particular waypoint of the flight plan, which time is commonly designated by the acronym RTA standing for “Required Time at Arrival”; this leads it to determine the speed strategy of the aircraft. The FMS regularly calculates an estimated time of arrival at the said particular waypoint, which time is commonly designated by the acronym ETA standing for “Estimated Time at Arrival”. If the estimated time of arrival departs by a predetermined tolerance with respect to the required time of arrival, a new cycle of calculations takes place, leading the FMS to redefine the trajectories to be followed by the aircraft as well as the speed strategy.
In the subsequent description and in the claims, the expression “required time of arrival” is understood logically to mean a time at which the aircraft must reach a particular point of its flight plan. The expression “estimated time of arrival” is understood to mean a time at which the FMS of the aircraft plans to reach the said particular point, taking account of the current speed of the aircraft and weather conditions, for example.
The concept of time constraint can consist of a tolerance in relation to the required time of arrival. The tolerance is generally modelled in the form of a funnel, that is to say it is increasingly narrow as the aircraft approaches the particular waypoint. Indeed, on approaching the said particular waypoint, compliance with the required time of arrival demands greater and greater precision. However, other ways of defining the time constraint on the required time of arrival exist, as is described further on in the description.
Moreover, each aircraft exhibits a speed envelope, comprising a maximum speed profile and a minimum speed profile, the said maximum and minimum speeds being able to vary as a function notably of the altitude and the weight of the aeroplane. This speed envelope defines a range of speeds that can be reached by the aircraft; current FMSs therefore define the speed strategy inside this speed envelope.
Now, it may happen that the FMS determines a speed strategy for which the speed supposedly making it possible to adhere to the time constraint departs from the mean of the maximum and minimum speeds of the speed envelope of the aircraft. The aircraft's speed guidance module may thus be led to define a setpoint speed which approaches the bounds of the aircraft's speed envelope. This situation reduces the margin of manoeuvre available to the aircraft in terms of speed. This constitutes a major problem since, as a function of the vagaries of the flight, and of inaccuracies, if any, in the calculations of predictions, the absence of margin on the speed may make it impossible to adhere to a time constraint. Indeed, the intrinsic capabilities of the aircraft, and the consideration of the environment, notably the weather, allows the FMS to estimate a maximum time of arrival after which the aircraft cannot arrive, and a minimum time of arrival before which the aircraft cannot arrive. If the required time of arrival approaches one of these extreme values, the risk of not adhering to the time constraint increases.
Currently, aircraft operators have a tendency to construct by themselves a margin of manoeuvre allowing them to adhere to their time constraints. However, this empirical approach does not afford a sufficient guarantee; moreover, it has a tendency to induce changes of speed of significant amplitude, sources of discomfort to the passengers and of overconsumption of fuel.
An aim of the invention is notably to alleviate this drawback. Thus, to ensure the maintenance of a sufficient margin, the method for aiding the formulation of the speed strategy of an aircraft according to the invention makes it possible to anticipate the risks of reaching the limits of the aircraft in terms of achievable required time of arrival.